The present invention relates to controllers for serial printers, for use with electronic calculators and the like, using a DC motor as a drive source and printing information by moving a print head in a digit direction.
As disclosed in Japanese Patent Unexamined Publications Nos. 57-182472/(1982) and 58-112781/(1983), controllers of this type are widely used in serial printers which print information by causing a carriage to select a character pattern to be printed from the inner side of a plurality of character wheels, each having a plurality of characters. The carriage moves in a digit direction. An example of such serial printer 100 is shown in FIG. 8.
The conventional systems will be described with reference to FIGS. 9-16. FIG. 9 is a schematic diagram of the serial printer, FIG. 10 is a perspective view of a character group switching mechanism, FIGS. 11, 13 and 15 are timing charts of the serial printer, and FIGS. 12, 14 and 16 are graphs showing behaviors of mechanical parts of the serial printer. Since the serial printer described with reference to the invention is disclosed in Japanese Patent Unexamined Publication No. 58-145472/(1983), the operation thereof be described only briefly.
Rotation in a direction A of a DC motor 1 is transmitted from a motor gear 2 to a reducing gear 3 and a planetary gear 4, and then distributed therefrom to a print switching gear 5 and a selection drive gear 16. The thus-transmitted rotational motion of the print switching gear 5 is further coupled to a print gear 6 to cause a print shaft 7 to rotate in a direction B, whereas the rotational motion of the selection drive gear 16 is further coupled to a selection gear 17 to cause a character wheel shaft 20 to rotate in a direction C. Switching between the print shaft 7 and the character wheel shaft 20 is effected by stopping the rotation while causing a selection pawl 18 to be engaged with either the print switching gear 5 or the selection gear 17. The selection pawl 18 is driven by an electromagnet 19.
A print cam 9 is mounted on the print shaft 7. The print cam 9 is axially slidable, rotates in phase with the print shaft 7, and is disposed within a carriage 8. When the print cam 9 makes a single full rotation, a hammer 11 is driven through a hammer transmission lever 10, the hammer biases a character wheel 14 inked by an ink roll 12 mounted on a character wheel body 13, whereby information is printed on a sheet 15. Upon printing, the position of the carriage is displaced to an upper digit by engagement of a carry lead 9a with a positioning plate 25. The character wheel body 13, a character clutch 29, an outer clutch 27, an intermediate clutch 28, and a detecting wheel 21 are arranged on the character wheel shaft 20. The character wheel body 13 and the character clutch 29 are axially slidable and rotate in phase with the character wheel shaft 20. The outer clutch 27 and the intermediate clutch 28 are rotatably supported irrespective of the rotational phase of the character wheel shaft 20. The detecting wheel 21 rotates integrally with the character wheel shaft 20. The character wheel body 13 holds character groups 14a, 14b, 14c. The rotational phase of the character wheel body 13 is detected by a timing pulse detected by a detecting brush 22 and a reset pulse (hereinafter referred to as "RP") detected by a detecting brush 23. The detecting brushes 22, 23 slidably contact the detecting wheel 21. Since the character wheel 14 described in this embodiment is divided into 14 segments, 14 timing pulses are generated in one full rotation of the detecting wheel 21. The carriage 8 is biased by a spring 24 in a direction D. The character wheel 14 and the character clutch 29 are biased by a character wheel spring 26 and a clutch spring 30 in the direction D and a direction E, respectively.
Upon application of a motor drive signal a, counting of the number of timing pulses b is started while being reset to zero after a reset pulse c. A character selection signal d is applied at the number of timing pulses corresponding to a desired character to print such desired character. The time interval between the rise of a first digit character selection signal d and the fall thereof is measured by a timer (not shown) that is activated at the rise of the character selection signal d. Such measurement is made to allow the carriage of the serial printer described in this embodiment to return by a carriage return start conduction that adds a multiple of the interval during which the character selection signal d is turned on to the application of the character selection signal d.
A carriage operation f is performed after the character selection signal d has been applied. A character wheel body operation g is interlocked therewith up to a character group selection digit. After the character group has been selected, the carriage 8 is displaced relative to the character wheel body 13 by the distance at which each character group is arranged. The carriage operation f is again interlocked with the character wheel body operation g.
At this point, the timing pulse to be generated in correspondence to the character wheel that starts rotating again is a timing pulse that corresponds to a character next to a character selected for printing.
Hereinbelow, the operation of selecting the character groups 14a, 14b, 14c will be described with reference to FIGS. 9 and 10. Since the pawl 8a of the carriage 8 and a notch 27a of the outer clutch 27 are engaged with each other up to a predetermined digit, the hammer 11 mounted on the carriage 8 is positioned relative to the character group 14a (see FIG. 10). In the meantime, the intermediate clutch 28 is rotated by the character clutch 29 in phase with each other. At the character group selection digit, a character is selected at a position at which either a projected portion 28a or a recessed portion 28b of the intermediate clutch 28 confronts the pawl 8a of the carriage 8, and rotation of the character wheel shaft 20 is stopped. When the carriage is displaced to an upper digit, the outer clutch 27 is rotated by a cam (not shown) in a direction F. Accordingly, the pawl 8a is disengaged from the notch portion 27a, moving the character wheel body 13, the outer clutch 27, and the intermediate clutch 28 in the direction D by the force of the character wheel spring 26. When the pawl 8a is engaged with the recessed portion 27a of the outer clutch 27, the hammer 11 confronts the character wheel 14c, whereas when the pawl 8a is engaged with the projected portion 28a of the intermediate clutch 28, the hammer 11 confronts the character wheel 14b.
Generation of timing pulses is started again as the character wheel, which has been stopped during printing before completing the engagement of the pawl 8a with the recessed portion 27a of the outer clutch 27 or with the projected portion 28a of the intermediate clutch 28 after the character group has moved, starts rotating.
It is apparent, even without reference to FIG. 12 (described below), that the time required for displacement is longer when the character wheel 14c, whose displacement is larger, is selected than when the character wheel 14b, whose displacement is smaller, is selected.
Further, the character wheel body 13, the outer clutch 27, and the intermediate clutch 28 slide over the character wheel shaft 20. Since the lateral pressure applied from the character wheel shaft 20 increases in proportion to the rotational speed of the character wheel shaft and the sliding resistance due to friction increases with increasing rotational speed of the character wheel shaft, the time required for displacing the character groups is increased.
Further, at low temperatures, the viscosity of a lubricant applied to the sliding portion of the character wheel body 13 and the character wheel shaft 20 increases, which in turn increases the sliding resistance. As a result, the time required for displacing the character groups is increased.
Similarly, as the printer is used more frequently, the sliding portion between the character wheel body 13 and the character wheel shaft 20 not only becomes contaminated by ink, sheet powder, or the like, but the lubricant also becomes deteriorated, thus increasing the sliding resistance and increasing the time required for displacing the character groups.
A character selection signal d for the next digit after the desired character group has been selected is applied after counting a predetermined number of timing pulses N1 preset as a single number of timing pulses allowing both the hammer 11 and a desired character group to be displaced to a position at which they confront each other correctly (hereinafter referred to as "the number of character group slide timing pulses").
In the character wheel body operation g in FIG. 11, the operation of selecting the character group 14c when the interval between timing pulses is short (at a high motor speed) is designated by n, the operation of selecting the character group 14b when the interval between timing pulses is short is designated by q, and the operation of selecting the character group 14c when the interval between timing pulses is long is designated by m.
FIG. 12 is a graph showing the number of character group slide timing pulses in relation to the timing pulse interval. The "+" mark designates the worst value out of data obtained in each timing pulse interval required for moving the character wheel body 13 from the character group 14a to the character group 14c, whereas mark "o" designates the value required for moving the character wheel body 13 from the character group 14a to the character group 14b.
From this data, a value N2 has been set as the number of wait timing pulses. The number of wait timing pulses is a time between a character group slide completion after a character group selection conduction and a next digit selection is ready. The number of wait timing pulses, i.e., N2, has been selected by considering variations based when the number of character group slide timing pulses is maximized, i.e., when the character wheel 14c is selected and when conditions such as ambient temperature, power supply voltage affecting the rotational speed of the character wheel shaft, or the rotational speed of a DC motor, and frequency of use, are the worst.
When the character selection has been ended at the most significant digit, a carriage return conduction j shown in the character selection signal d in FIG. 13 is effected. As a result, the carry lead 9a of the print cam 9 is disengaged from a projection 25a of the positioning plate 25, the carriage 8 and the character wheel body 13 are displaced in the direction D to return to the home position by the force stored in the character wheel spring 26 and the carriage spring 24. Unless the number of print digits is so small as 2 or 3, the character wheels that have been stationary during printing begin rotating before the carriage completely returns to the home position. As a result, generation of timing pulses is started again.
While the character wheel body 13 supporting the character wheels slides over the character wheel shaft 20, the lateral pressure applied by the character wheel shaft 20 increases in proportion to the rotational speed of the character wheel shaft 20. Thus, the larger the rotational speed of the character wheel shaft, the larger the sliding resistance due to friction. As a result, the time required for the carriage to return increases as well. At low temperatures, the viscosity of the lubricant applied to the sliding part of the character wheel body 13 and the character wheel shaft 20 increases, which in turn increases the sliding resistance. As a result, the time required for the carriage to return to the home position increases as well.
Similarly, as the printer is used more often, the sliding part between the character wheel body 13 and the character wheel shaft 20 not only is contaminated by ink, sheet powder, or the like, but the lubricant also deteriorates, thus increasing the sliding resistance as well as the time required for the carriage to return.
When printing subsequent lines continuously, the character selection signal d is applied after the preset number of timing pulses in which the carriage can make a complete return to the home position has been counted.
As described above, the time required for the carriage to return to the home position differs depending on the number of printed digits (the distance to be returned), the ambient temperature, the power supply voltage determining the rotational speed of the character wheel, i.e., the rotational speed of the DC motor, and the frequency of use. Therefore, the value that permits printing of the most significant digit and printing under the worst conditions in terms of temperature, power supply voltage, and frequency of use, has been selected as the time required for the carriage to return.
FIG. 14 is a graph showing the number of carriage return timing pulses as a function of the timing pulse interval. Mark "+" designates the worst data obtained from each timing pulse interval.
From this data, a value N3 has been set as the number of wait timing pulses. The number of wait timing pulses is a time between a carriage return completion after a carriage return conduction j and a next line character selection is ready. The number of wait timing pulses, i.e., N3, has been selected by considering variations based on the maximum number of timing pulses generated before the carriage returns to the home position (hereinafter referred to as "the number of carriage return timing pulses"), as shown in FIG. 13.
FIG. 16 is a graph showing the number of carriage return timing pulses as a function of temperature. Mark "+" designates the worst data obtained from each temperature. From this data, the value N2 has been used as the number of wait timing pulses, which is a time between a carriage return completion after a carriage return conduction j and a next line character selection is ready, by considering variations based on the maximum number of timing pulses generated before the carriage returns to the home position, as shown in FIG. 15.
However, in the conventional systems, the number of wait timing pulses, or wait time, between a carriage return completion in which the carriage returns in the digit direction and a next line character selection ready has been set to a constant value, which does not depend on the number of printed digits. For this reason,
(1) even if the carriage could return to the home position within a shorter time because the number of printed digits is small and thus the return distance is short, the carriage must wait for a time longer than required, thus increasing the time required for printing a single line of characters, i.e., slowing the print speed.
FIG. 3 is a part of a flowchart showing the operation of an exemplary controller of a conventional printer. The controller is designed to count in Step P3a a constant time required for printing the largest number of digits, not depending on the number of digits actually printed, as the number of timing pulses to be generated by the printer for control upon completion of printing a single line of characters. FIG. 4 is a portion of a flowchart showing the operation of another conventional controller in which a timer provided in a control circuit (including a program) is used to measure a constant return wait time in Step P4a. Again, it is a constant time required for printing the largest number of digits, not depending on the number of digits actually printed, that is counted as a wait time to a next line character selection ready timing.
FIG. 5 shows the breakdown of print times by a conventional controller. Carriage return wait times (e.g., the same as the number of wait timing pulses) P5a, P5b, P5c among print times for a single line are constant time periods required for printing the largest number of digits, and thus do not depend on the number of digits actually printed. Thus, the smaller the number of digits printed, the larger the percentage of the carriage return wait time. Additionally, the smaller the number of digits printed, the longer the carriage return wait time becomes than is required, as described before.
Further, according to the conventional systems, the number of next digit selection wait timing pulses between character group selection and selection completion is selected not only on based on the maximum number of character group slide timing pulses at the maximum rotational speed of the motor at the highest voltage (e.g., the minimum timing pulse interval), but also in consideration of conditions at low temperatures at which the character group slide time increases. The durability of the motor is taken into account in the maximum rotational speed of the motor. For this reason,
(2) the number of wait timing pulses is set to a value far larger than is required under the normal conditions, thus preventing the print speed from being improved.
To overcome this problem, the character group slide time must be curtailed. If the load of the character wheel spring is set to a large value, the printer load is increased, thereby increasing costs due to high power consumption and expensive heavy-duty parts. Further, the printer is noisier during carriage return.
Further, the controller for a conventional serial printer has regularly set the next digit selection wait time to character group slide completion irrespective of the character group slide time that differs from one character group to another. For this reason,
(3) a character group that is used most frequently is located at a position for which a small number of character group slide timing pulses is set. Therefore, even if the character group is selected and printed, the print speed is not improved.
Further, when the serial printer is employed in an electronic calculator, or the like, an alkali battery or an inexpensive low-capacity ac adaptor is used as a power source thereof. The battery is seldom used at the maximum voltage, but instead is usually used at a voltage lower than the center (i.e., intermediate) voltage. The ac adaptor is usually used at a voltage lower than the center voltage during printer driving to balance low power consumption for calculation (e.g., high output voltage) with high power consumption for the printer driving (e.g., low output voltage).
Hence, the number of next line character selection wait timing pulses before return completion in the conventional systems is selected in consideration of variations based on the maximum number of carriage return timing pulses at the maximum rotational speed of the motor at the highest voltage (e.g., the minimum timing pulse interval). The durability of the motor is taken into account in the maximum rotational speed of the motor. For this reason,
(4) the number of wait timing pulses is set to a value far larger than required under the normal conditions, thus preventing the print speed from being improved.
Since the electronic calculator including the serial printer is used over a wide temperature range, the number of next digit character selection wait timing pulses to carriage return completion is set based on the worst conditions at low temperatures in the above-mentioned conventional systems. For this reason,
(5) the number of wait timing pulses is set to a value far larger than required under the normal conditions, thus preventing the print speed from being increased.
To overcome this problem, the character group slide time must be curtailed. As mentioned above, if the load of the character wheel spring is set to a large value, the printer load is increased, thereby increasing the printer costs due to high power consumption and expensive heavy-duty parts. Further, it is noisier than before during carriage return.
To overcome the above problem by increasing the operation speed of the printer, a motor with a larger torque is necessary. This increases the shape, weight, and price of the motor, thereby increasing the shape, weight, and price of the serial printer.
The invention has been made to overcome the above problems. Accordingly, an object of the invention is to provide a controller for a serial printer whose speed is high under often used characters, temperatures, and power supply voltages, without increasing the shape, weight, power consumption, price, or noise of the printer, and without using heavy duty (e.g., highly durable) parts.